1. Field of the Invention
The present invention relates to a high-frequency amplifier circuit in a receiver used for radio communication and so on.
2. Description of the Related Art
In a receiver used for radio communication and so on, a lower power consumption in a high-frequency amplifier circuit needs to be achieved in order to extend the life of a battery. As such a high-frequency amplifier circuit, a two-stage amplifier circuit in which two transistors are connected to each other in cascade has been proposed (Japanese Patent Application Kokai No. 2003-243938, for example). In the two-stage amplifier circuit, a current flowing through the transistor in the second stage is reused in the first stage. Thus, a power consumption in the circuit as a whole can be kept low.
In such a receiver, a signal input from an antenna is amplified by a high-frequency amplifier circuit and supplied to a frequency conversion circuit. The frequency conversion circuit mixes the signal supplied by the high-frequency amplifier circuit with a local signal supplied by a local oscillation circuit for conversion into an intermediate frequency. The frequency conversion circuit supplies the obtained intermediate-frequency signal to a demodulation processing unit. At this time, a phenomenon in which part of the local signal applied to the frequency conversion circuit by the local oscillation circuit flows not in a direction toward the demodulation processing unit but in a direction heading toward the antenna from the high-frequency amplifier circuit, i.e., in a reverse direction, occurs. If such a signal flowing in the reverse direction reaches the antenna, unwanted radiation occurs, resulting in a cause of electromagnetic interference. Thus, such a signal needs to be blocked in the high-frequency amplifier circuit as much as possible.
In order to improve a characteristic of blocking such a signal flowing in the reverse direction (reverse isolation), it is preferable that parasitic capacitance from an output to an input is made small by connecting a plurality of transistors using connection structure such as cascade structure. However, if the number of transistors connected in cascade structure and so on is increased, a power-supply potential needs to be set high, resulting in a higher power consumption.